People have different eye colors due to variations in the amount and quality of melanin pigments in the iris of their eyes. Melanin is produced by cells called melanocytes and determines the color of a person’s eyes, skin, and hair. The two main factors that influence eye color are the pigmentation of the iris epithelial layer caused by melanin content, and the scattering of light by the turbid medium in the stroma of the iris.
Melanin Production
Melanocytes produce two types of melanin: eumelanin which produces brown and black hues, and pheomelanin which produces red and yellow hues. The mixture and ratio of eumelanin and pheomelanin determines the base eye color of an individual. More eumelanin means darker eyes, while more pheomelanin means lighter eyes.
Genetics of Eye Color
Eye color is a polygenic phenotypic character determined by 2 distinct factors:
– The pigmentation of the iris, which is determined by the amount and type of melanin
– The Rayleigh scattering of light in the stroma of the iris, which is determined by the density and composition of extracellular matrix components
It is estimated that at least 16 different genes play a role in determining eye color in humans, with the main genes being:
| Gene | Chromosome | Associated Eye Colors |
| OCA2 | 15q | Brown/blue |
| HERC2 | 15q | Blue/green/hazel |
| SLC24A4 | 14q | Blue/green/hazel |
| TYR | 11q | Green/brown/blue |
| SLC45A2 | 5p | Brown/blue/green |
The key genes involved in eye color are OCA2 and HERC2. The OCA2 gene encodes the P protein which is involved in the maturation and transport of melanosomes, the organelles that produce and store melanin. The HERC2 gene regulates expression of OCA2. Different variations in these genes reduce melanin production leading to lighter eye colors.
Blue Eyes
Blue eyes are caused by an absence of melanin pigmentation in the iris stroma. This allows scattering of short-wavelength light, especially light in the blue spectrum, to predominate.
Blue eyes are recessive, meaning both parents must carry the gene mutation for a child to have blue eyes. The gene mutation reduces the expression of OCA2 leading to less melanin production. Most blue-eyed people have the “TGTG” variation in the HERC2 gene.
Geographically, blue eyes are most commonly found in people of Northern and Eastern European ancestry.
Brown Eyes
Brown eyes are caused by large amounts of melanin in the iris, especially eumelanin pigments. The OCA2 gene is non-mutated, which allows extensive melanin production. The large amount of eumelanin absorbs most light wavelengths, causing the brown coloration.
Brown is the most common eye color worldwide, especially in Asia and Africa. This is thought to be adaptive, as melanin provides some protection against sun damage. Increased melanin may have been evolutionarily advantageous in areas receiving higher UV radiation.
Green Eyes
Green eyes have low to moderate amounts of melanin. They contain more pheomelanin pigment than melanin pigment. The hazel green hue is a result of Rayleigh scattering of medium wavelength light. Green eyes are uncommon globally but can be found in Northern and Central Europe.
Hazel Eyes
Hazel eyes appear greenish-brown or yellowish-brown. They feature a concentration of melanin at the boundary between the iris and the sclera, framing the iris. A mixture of Rayleigh scattering and moderate melanin levels produce the multi-colored hazel appearance.
Gray Eyes
Gray eyes have the least melanin and are considered a darker shade of blue eyes. They are more common in Eastern European countries like Russia. The gray color results from larger collagen deposits in the stroma.
Amber and Red Eyes
Amber eyes feature a solid orange or copper color and are quite rare. True red eyes are caused by albinism, whereby the iris lacks any pigmentation allowing blood vessels to show through. Both these eye colors indicate higher levels of pheomelanin.
Heterochromia
Some people display heterochromia or different colored eyes between their two irises. This can be due to uneven melanin deposition, trauma, or specific syndromes like Waardenburg syndrome. Most cases of heterochromia are benign.
Eye Color Changes with Age
Infants are usually born with light blue or gray eyes as melanin deposition is still low. Eye color is established by 3 years of age as melanocytes ramp up melanin production. Eye color can darken into adolescence and then may lighten again with age as melanin levels decrease.
Conclusion
In summary, the main factors influencing eye color are:
– Genetics – Specific variations in OCA2 and HERC2 genes reduce melanin production and lead to lighter eye colors.
– Melanin content – Higher melanin levels produce brown/black eyes, lower levels produce green/hazel/blue eyes.
– Rayleigh scattering – The degree of light scattering in the iris stroma also impacts eye color.
Eye color is a polygenic trait determined by the expression of at least 16 different genes. The most important genes are OCA2 and HERC2 which regulate melanin synthesis. Geographic ancestry, gender, and age can also influence melanin levels and perceived eye color. Heterochromia demonstrates the complex genetic control of eye pigmentation.
